Vitamin receptors are overexpressed on cancer cells. For example, the folate receptor, a 38 KD GPI-anchored protein that binds the vitamin folic acid with high affinity (<1 nM), is overexpressed on many malignant tissues, including ovarian, breast, bronchial, and brain cancers. In particular, it is estimated that 95% of all ovarian carcinomas overexpress the folate receptor. In contrast, with the exception of kidney, choroid plexus, and placenta, normal tissues express low or nondetectable levels of the folate receptor. Most cells also use an unrelated reduced folate carrier to acquire the necessary folic acid.
Following receptor binding of vitamins such as folate to vitamin receptors, rapid endocytosis delivers the vitamin into the cell, where it is unloaded in an endosomal compartment at lower pH. Importantly, covalent conjugation of small molecules, proteins, and even liposomes to vitamins and other vitamin receptor binding ligands does not block the ability of the ligand to bind to its receptor, and therefore, such ligand conjugates can readily be delivered to and can enter cells by receptor-mediated endocytosis.
It has also been shown that activated monocytes overexpress the folate receptor. The overexpression of folate receptors on activated macrophages, and on activated monocytes, is described in U.S. Patent Application No. 60/696,740 and U.S. Patent Application Publication No. US 2002/0192157, each entirely incorporated herein by reference. Further, it has also been reported that the folate receptor β, the nonepithelial isoform of the folate receptor, is expressed on activated, but not resting, synovial macrophages. Activated macrophages can participate in the immune response by nonspecifically engulfing and killing foreign pathogens within the macrophage, by displaying degraded peptides from foreign proteins on the macrophage cell surface where they can be recognized by other immune cells, and by secreting cytokines and other factors that modulate the function of T and B lymphocytes, resulting in further stimulation of immune responses. However, activated macrophages can also contribute to the pathophysiology of disease in some instances. For example, activated macrophages can contribute to atherosclerosis, rheumatoid arthritis, autoimmune disease states, and graft versus host disease, among other disease states.
An example of the contribution of activated macrophages to disease states is the involvement of activated macrophages in the progression of atherosclerosis. Atherosclerosis is a disease state initiated when a fatty streak forms within a blood vessel wall. Formation of fatty streaks is believed to result from accumulation of lipoprotein particles in the intima layer of the blood vessel wall, the layer of the vessel wall underlying the luminal endothelial cell layer. Lipoprotein particles can associate with extracellular matrix components in the intima layer and can become inaccessible to plasma antioxidants, resulting in oxidative modification of the lipoprotein particles. Such oxidative modification may trigger a local inflammatory response resulting in adhesion of activated macrophages and T lymphocytes to the luminal endothelium followed by migration into the intima layer. The oxidized lipoprotein particles themselves can act as chemo-attractants for cells of the immune system, such as macrophages and T cells, or can induce cells in the vascular wall to produce chemo-attractants. The atherosclerotic lesion may then form a fibrous cap with a lipid-rich core filled with activated macrophages. Atherosclerotic lesions that are unstable are characterized by local inflammation, and lesions that have ruptured and have caused fatal myocardial infarction are characterized by an infiltration of activated macrophages and T lymphocytes.
U.S. Pat. No. 6,782,289, U.S. Patent Application Publication No. US 2005/0244336, and PCT International Publication No. WO 2004/110250, each entirely incorporated herein by reference, provide discussions of possible origins of blood vessel disease. The references disclose catheter-based systems for detection of radio labeled conjugates that bind to activated macrophages within a blood vessel or other body lumen.